1. Field of the Invention
The present invention relates to a 3-phase input type of switching power circuit.
2. Discussion of Background
The best known type of switching power circuit in the prior art is a single phase input type of switching power circuit. In a single phase input type of switching power circuit, a single phase AC power is converted to DC power with a rectifying and smoothing circuit of the capacitor input type. Switching is performed and the converted power is output through a rectifying and smoothing circuit as DC power to be applied to a load. Although the single phase input type of switching power circuit has the advantage of simplicity, it presents various problems. For instance, since the electric current for charging that runs to the capacitor used in the input smoothing circuit is pulsed, the input power factor, at, for example, approximately 70%, is poor. Also, the higher harmonics component in the electric current is large and this causes a number of problems in the power system. Furthermore, a rush current runs through the input smoothing capacitor, sometimes causing a momentary reduction in the source voltage and an electrical breakdown in the rectifying circuit.
Known technology designed to solve the problems described above is disclosed in Japanese Unexamined Patent Publication No. 207268/1991. The technology disclosed in this publication employs a dither rectifying circuit. The dither rectifying circuit generates a composite signal in which a high frequency wave is superimposed on a sine wave by adding a high frequency dither signal to a non-linear current signal which runs to the input smoothing capacitor. The sine wave component is extracted from this composite signal using a low pass filter. With this, the power factor is significantly improved.
However, Japanese Unexamined Patent Publication No. 207268/ 1991 discloses only a single phase input type of switching power circuit and it does not disclose a 3-phase input type of switching power circuit.
To hypothesize a 3-phase input type of switching power circuit achieved with the technology for a single phase input type of switching power circuit disclosed in the prior art, it will be necessary to provide at least 3 switching elements for the sine wave input converter unit and another switching element for the DC-DC converter unit, for a total of four switching elements. In addition, 3 input reactors and one transformer for the converters are required. This means that the number of required components is high.